1. Field of The Invention
The invention relates to a data transmission and reception apparatus and method for selecting two modes in which the bit numbers of one word are different but the sampling frequencies are the same.
2. Description of The Prior Art
In the case of an 8-mm VTR, an audio signal is recorded by mixing it with a color video signal in such a manner that the audio signal can be separated in terms of frequency using frequency modulation, and as an option, the audio signal can be subjected to pulse code modulation (PCM) and separated, in terms of area, from the color signal, and the recording can be made on one track formed by both signals.
FIG. 1 shows one example of a rotary head device of an 8-mm VTR, and FIG. 2 shows its tape format.
In FIG. 1, HA, HB are rotary magnetic heads for recording and reproducing, and these heads HA, HB are made so that the azimuth angles of the head gaps are different from each other, mounted at angles of 180.degree. with respect to each other, and rotated in the direction of the arrow 3H at a frequency (30 kHz) in which they slightly project from the circumference of a drum 1. A magnetic tape 2 is attached over an angular range of 221.degree. with respect to the circumference of the drum 1 and run at a constant speed in the direction indicated by the arrow 3T.
Therefore, tracks 4A and 4B having a length corresponding to 221.degree. are formed alternately on the tape 2 by the rotary heads HA and HB as shown in FIG. 2 and a signal is recorded. In an area AP corresponding to an angle range of about 36.degree. from the time when the rotary heads HA and HB of the tracks 4A and 4B (which contain a margin for after-recording a PCM audio signal and a guard band for distinguishing between area AP and area AV) start their scanning, a PCM audio signal associated with one field of a video signal is recorded in the time-compressed state. In a subsequent area AV corresponding to an angle range of 180.degree., a color video signal of one field, an FM audio signal and a signal for tracking are recorded. The FM audio signal is mixed in a lower frequency band of the color video signal. The remaining 5.degree. is left for an allowance period for the separation of the heads from the tape. FIG. 3 shows a track format for recording a PCM audio signal on areas AP2.about.AP6 respectively which are formed by dividing area AV (FIG. 2) by every 36.degree. rotation of the magnetic heads HA and HB, thereby enabling recording of six audio channels in total on the areas AP1.about.AP6.
The aforementioned recording system as shown in FIGS. 2 and 3 is described in detail in U.S. Pat. No. 4,542,419. The PCM audio signal in such an 8-mm VTR is made to have one word/8 bits and is slightly inferior in terms of performance as compared with CD (compact disc) or DAT (digital audio tape).
Recently the performance of magnetic tape as a recording medium has been improved, and without extending the PCM area AP of the existing angle range of 36.degree., audio data, each word of which is composed of 12 bits, can be recorded at a sampling frequency of 48 kHz with respect to a metal powder tape (MP tape) with metal powder bounded, and audio data, each word of which consists of 16 bits, can be recorded at the sampling frequency of 48 kHz with respect to a metal evaporated tape (ME tape).
Therefore, the bit number per word of the audio data can be increased. In this case, it is convenient for a user to be able to select between either of two kinds of modes for recording audio data to match the tape performance: one word/12 bits in the PCM area AP (MP mode) or one word/16 bits (ME mode).
However, in a system where the selection between the two kinds of modes is possible, separate processing systems for one word/12 bits of data and the one word/16 bits of data must be provided. This causes an impractical increase of hardware. For this reason, it is desired that the requirement for such increased hardware be reduced as much as possible.
Certain known DATs (Digital Audio Tape Recorders) are constructed so that they can assume a one word/16 bits mode and a one word/12 bits mode. The sampling frequency f.sub.s is 48 kHz with the one word/16 bits mode. However, with the one word/12 bits mode, the sampling frequency f.sub.s 32 kHz and the speed of the magnetic tape and the rotation number of the rotary head drum are reduced by 1/2 to enable long-hour recording. Specifically, although the bit period of the serial data is 13 .mu.sec in the case of f.sub.s =48 kHz and one word/16 bits, the bit period of the serial data is 26 .mu.sec in the case of f.sub.s =32 kHz, and one word/12 bits. If the speed of the magnetic tape and the drum speed are reduced by 1/2, the recording wavelength, i.e., the transmission bit rate per bit on the tape can be kept identical.
In the DAT, a hardware increase is prevented by using the error correction encoder, decoder, etc. in common in the two modes as follows.
FIG. 4 shows a code structure of a PCM audio signal and redundant data of the error correction code which are recorded on one segment formed at one scanning of the rotary head. Each column shows one block, and M blocks are arranged side-by-side in the horizontal direction. A PCM audio signal in one block is composed of N words, and NXM words of PCM audio signals are arranged in a two dimensional array. An error detection code C1 is added to each block in the longitudinal direction of the two dimensional array of the audio PCM signals, and an error correction code C2 is added in the horizontal direction. An n-word check code P of the error detection code C1 is contained in each block. Similarly an m-word check code Q of the error correction code C2 is added to each row of M blocks.
In this case, the sizes of the two dimensional planes on which the encoding of C1 and C2 is done are the same for a one word/16 bits mode and a one word/12 bits mode. Also, the length of one data block, which is one code sequence of the code C1, is selected to be the least common multiple L of 16 bits and 12 bits, which is multiplied by an integer, and the bit numbers per block are made equal in both modes. In the case of the one word/16 bits mode, the 16-bit word is composed of an upper eight-bit symbol and a lower eight-bit symbol to form an error correction code, and in the case of the one word/12 bits mode, as shown in FIG. 5, exactly the same error correction encoding process as in the case of the one word/16 bits is performed after reallocating its upper eight bits and lower four bits.
With the above-mentioned technique, the common use of the error correction encoder and the error correction decoder is achieved in both modes of one word/16 bits and one word/12 bits to prevent a hardware increase (refer to U.S. Pat. No. 4,688,225 and U.S. Pat. No. 4,758,907).
Therefore, it is possible in an 8-mm VTR to use a conversion table for 16 bits and 12 bits of DAT data when the recording and reproduction of audio data of both the ME mode (one word/16 bits) and the MP mode (one word/12 bits) are made in the PCM audio area AP corresponding to the ME tape or the MP tape.
However, in the case of the 8-mm VTR, the sampling frequency of the audio signal, transporting speed of the magnetic tape and rotational speed of the rotary heads are not changed with a change of the modes. The recording is made in the PCM audio area AP at an adequate recording wavelength per bit, depending on tape types (ME/MP), and the transmission bit rate is not the same in both modes as in a DAT. As a result, the sizes of the two dimensional planes of the PCM audio signal are different in both modes.
For example, FIGS. 6A and 6B show one example of a two dimensional plane construction of error correction blocks of a one word/16 bits mode and a one word/12 bits mode for NTSC adaptation, respectively. The planes are stored in a memory. Numerals indicated in the drawing show the byte number (one byte=8 bits=1 symbol), each column in the longitudinal direction is one block having (4+40+4=48 bytes=48 symbols=8 symbols+20 words), and this is maintained unchanged in both modes. Consequently, the circuit for adding the block synchronization signal or block address data can be made to have the same structure in both modes. In the drawing, a header composed of the block synchronization signal and block address data is indicated in the state where the header is added to each block. The detail of the construction of the block shown in FIG. 6A is described in the U.S. patent application Ser. No. 252,807 filed on Sept. 30, 1988 by the same assignee. Each block in the case of the one word/12 bits is handled as symbol data for every eight bits by a conversion table in a manner similar to the above-mentioned DAT.
In this way, if the length of each block in the longitudinal direction of the two dimensional array of PCM audio signals is selected to have a common value (48 bytes in the drawings) for the one word/16 bits mode and the one word/12 bits mode, the byte numbers in the horizontal direction become different since the word numbers (not bit numbers) contained in the two dimensional planes are the same in both modes. However, with the following method, it is possible to keep the construction of the error correction code unchanged.
Specifically, the number of symbols of audio data in the horizontal direction is 80 bytes in the 16-bit mode in this example, and its number is selected to be 12/16=3/4 of 80 bytes=60 bytes in the 12-bit mode.
In the 16-bit mode, as shown by the circles in FIG. 36A, one sequence of the error correction code C2 in the horizontal direction is composed of symbols which lie every four bytes. Consequently one row in the horizontal direction has four sequences for the correction code C2. For example, one sequence of its code C2 is constructed by the (25, 20, 6) Reed-Solomon code having five-byte check code per one sequence. The terminology (25, 20, 6) means that there are 25 total symbols, 20 data, and the minimum length is 5 parity+1=6 bits. A check code Q of a total of 20 bytes for four sequences is added to the left-hand side of the PCM audio data per one row in the horizontal direction as shown in FIG. 6A.
In addition, in the 12-bit mode, as shown by circles in FIG. 6B, one sequence of the error correction code C2 is composed of symbols which lie every three bytes. Consequently one row in the horizontal direction has three sequences for the error correction code C2. One sequence of the code C2 is constructed with the (25, 20, 6) Reed-Solomon code in a manner similar to the 16-bit mode, and a five-byte check code Q is developed per one sequence. A check code Q of a total of 15 bytes is added to the left-hand side of the audio PCM data as indicated in FIG. 6B.
In this case, the byte number of audio data corresponding to one field period to be recorded in the area AP on a tape becomes ##EQU1## when the sampling frequency is 48 kHz, one word is 16 bits, and two channels for the left-and right-hand sides are employed in the presence of an NTSC signal. As a result, 3204 bytes are needed. However, in the case of FIG. 6A, 40.times.80=3200 bytes are needed to provide a remainder of 4 bytes. The four-bytes of data can be dealt with by adequately inserting it into the header. The technique described in the aforementioned U.S. patent application Ser. No. 252,807 can be employed for inserting four symbols into the header.
The error correction code C1 in the longitudinal direction is the same with the two modes, and the (44, 40, 5) Reed-Solomon code, for example, is used.
In the above manner, the structures of the error correction codes in the two modes can be made identical and a circuit for adding the header containing a block synchronization signal and address data can be used in common.
However, as clear from FIGS. 6A and 6B, the block structure of the two dimensional array for error correction is varied with the one word/16 bits mode and the one word/12 bits mode. For this reason, two kinds of format processes are needed depending on each mode. This imposes an extreme burden on the hardware.